Add chain exchange and partial message metrics

[masih]

Add metrics that measure various aspects of chain exchange and partial messages, including:

• The number of messages pending partial messages.
• The total number instances with a partial message.
• The chain exchange broadcast count.
• The length of broadcast chain.
• The time spent on chain exchange validation.
• Qualify existing GPBFT validation metrics with partial message attribute.
• The number of duplicate partial messages qualified with equivocation attribute.

Fixes: #812

[codecov]

Codecov Report

Attention: Patch coverage is 75.32468% with 19 lines in your changes missing coverage. Please review.

Project coverage is 67.10%. Comparing base (bae93e2) to head (cc97a49).

Files with missing lines	Patch %	Lines
partial_msg.go	0.00%	12 Missing ⚠️
chainexchange/pubsub.go	87.80%	5 Missing ⚠️
internal/measurements/attributes.go	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #852      +/-   ##
==========================================
- Coverage   67.21%   67.10%   -0.11%     
==========================================
  Files          83       84       +1     
  Lines        9055     9105      +50     
==========================================
+ Hits         6086     6110      +24     
- Misses       2430     2454      +24     
- Partials      539      541       +2     

Files with missing lines	Coverage Δ
chainexchange/metrics.go	100.00% <100.00%> (ø)
host.go	64.82% <100.00%> (-0.05%)	⬇️
metrics.go	90.62% <100.00%> (+3.12%)	⬆️
internal/measurements/attributes.go	83.33% <83.33%> (ø)
chainexchange/pubsub.go	72.41% <87.80%> (+1.16%)	⬆️
partial_msg.go	55.78% <0.00%> (-2.88%)	⬇️

... and 3 files with indirect coverage changes